Over the last ten years, Vehicular Ad Hoc Networks (VANETs) have received significant attention from the academic and industrial communities alike. VANETs are a particular type of mobile ad hoc network originally designed for the purpose of facilitating the creation of spontaneous wireless networks between different vehicles, but since their inception the scope of VANETs has been extended to other types of road users such as cyclists and pedestrians. Due to the volatility of the wireless medium, VANETs face several challenges, especially when applications with a diverse set of requirements must be supported. Among the various techniques used to address such challenges, one of the most recent is Software-Defined Networking (SDN), which, by clearly separating the data plane from the control plane, allows the implementation of traditional network control and management tasks on top of a logically centralized controller. In this work, we perform a systematic review of SDN techniques tailored to the VANET domain. More specifically, we first review the literature on VANETs and SDN from an architectural and communications requirement perspective, then we report on the most recent standardization efforts, and finally, we highlight the open research areas and the most important challenges in this domain.